"There are Iraqis, Turks and Europeans still inside" the IS pocket, along with Libyans, Egyptians and French nationals of North African origin, according to Afrin.

And crucially there are hundreds of civilians that the SDF and coalition say the jihadists are using as human shields.

Looking from SDF-held vantage points, AFP journalists have seen men and women moving inside the IS pocket.

The holdout forces cannot be engaged in battle "without a direct operation that would require air and artillery support," according to Nicholas Heras at the Center for a New American Security.

This onslaught "would ultimately kill almost everyone in its way", he says.

The SDF and coalition say the presence of civilians -- mainly jihadists' wives and children -- have slowed down operations.

"Our military strategy today is to evacuate civilians", according to Afrin.

"They are relatives of IS fighters, but... we consider them as civilians, even if they are the children or wives of jihadists," he adds.

The United Nations is concerned about "some 200 families, including many women and children, who are reportedly trapped" in the IS holdout.

- What other obstacles are there? -

Coalition spokesman Sean Ryan says air strikes have also been scaled back "due to battlefield conditions."

In comments emailed to AFP, Ryan says tunnels, improvised explosive devices and the threat of vehicle borne suicide attacks have slowed the advance.

The tunnels -- along with basements -- provide cover for diehard fighters, both in terms of holding out against attacks and launching surprise assaults.

Colonel Francois-Regis Legrier, who leads a French artillery unit supporting Kurdish forces, has said victory would come sooner if Western powers deployed 1,000 of their own seasoned fighters on the ground.

- Is anyone talking to IS? -

The Observatory reports negotiations have taken place between the SDF and jihadists, and that a deal appears to have been reached.

The jihadists are allegedly demanding safe passage out of the Baghouz pocket.

One source familiar with the offensive tells AFP that IS fighters are seeking safe passage to Idlib in northwestern Syria, a province held by rival jihadists.

IS "wants to take remaining civilians with them as human shields", this source says, adding that the SDF are not willing to discuss the matter.

AFP has not been able to confirm this with an SDF official, but one of its commanders says IS has no leverage to negotiate.

"They are besieged in a very tight area and they have no other choice but to surrender," this commander contends.